How to scale the edges along a curved surface
Creating custom orientations will help you in many situations where you have to move or scale many faces/edges/vertices along directions that are off the usual orientations.
However, if you only have one face like the one in your screenshot it is a bit easier. And if you have multiple ones pointing in different directions, creating a custom orientation for each of them might be very uncomfortable.
Instead you can select the face and set the orientation to Normal (which would also be used if you created a custom orientation) and then scale along the face normal's X or Y axis with simply using S &gt; X (or Y). You don't have to press the key for the axis twice - that will use Global orientation.
It's only important that you use the face selection instead of only selecting the opposing edges you want to scale, because this might result in an unwanted orientation (see picture on the left).